1. Field of the Invention
The present invention relates to a crystallized glass, a magnetic disc substrate using the crystallized glass, and a magnetic disc using the magnetic disc substrate.
2. Description of Related Art
Generally, predetermined thermal expansion coefficient, smooth surface, high strength and so on are required for a magnetic disc substrate. Particularly, in a use of the magnetic disc substrate, a glass having a thermal expansion coefficient of 60-90 .times.10.sup.-7 /k level at near room temperature is required. Moreover, in order to improve a record density of a hard disc, it is necessary to decrease a suspension amount of a magnetic head, and thus a highly smooth surface is required for the substrate. Further, a magnetic film decreases its characteristics if it reacts with alkali metal ion. Therefore, it is necessary to avoid strongly an intrusion of alkali metal ion from the substrate to the magnetic film.
In Japanese Patent Laid-Open Publication No. 11-16151 (JP-A-11-16151) and Japanese Patent Laid-Open Publication No. 11-16143 (JP-A-11-16143), it was disclosed a crystallized glass for the magnetic disc in which a main crystal phase was .alpha.-quartz phase and lithium disilicate phase. In Japanese Patent Laid-Open Publication No. 10-226532 (JP-A-10-226532), it was disclosed a crystal glass for the magnetic disc in which a chemical durability was improved.
In the known crystallized glass having cristobalite phase and lithium disilicate phase as the main crystal phase, or, in the known crystallized glass having .alpha.-quartz phase and lithium disilicate phase as the main crystal phase, crystal particles are liable to be large in both cases, and thus there arises a problem such that a surface roughness after a precise polishing treatment becomes not so smooth. Particularly, in the crystallized glass having .alpha.-quartz phase and lithium disilicate phase as the main crystal phase, a thermal expansion coefficient is about 100.times.10.sup.-7 /k, and thus it is too large to be used for the magnetic disc. Moreover, in the crystallized glass having cristobalite phase and lithium disilicate phase as the main crystal phase, or, in the crystallized glass having .alpha.-quartz phase and lithium disilicate phase as the main crystal phase, an elution amount of lithium from the substrate is large, and thus an affection applying to a magnetic film is worried about.
For example, in the above JP-A-11-16151 and JP-A-1 1-16143, an elution of lithium is not mentioned at all. Moreover, since .alpha.-quartz phase is precipitated as a condensed state, a surface roughness (Ra) after a precise polishing treatment is 3-9 angstrom at best. In the above JP-A-10-226532, a chemical durability of glass is described in relation to a specific composition and also it is explained with reference to a glass weight variation, but an elution of lithium is not described at all.